This invention relates to a non-aqueous electrolyte secondary battery employing an organic electrolyte, a polymer solid electrolyte or the like. In particular, it relates to a novel constitution of positive and negative electrodes having a higher capacity without sacrificing the charge-discharge cycle life.
A variety of secondary batteries including small-sized sealed lead-acid and nickel-cadmium systems have been developed as power sources for driving portable electronic equipment. To minimize the overall size and weight of such a driving secondary battery, newly introduced and marketed are nickel-metal hydride systems, lithium-ion systems, and other advanced secondary batteries, which are higher in the energy density. As their products are welcome widely in the market, lithium-ion batteries above all are focused which are substantially equal in the capacity density per unit volume (Wh/L) to nickel-metal hydride systems but almost two times higher in the capacity density per unit weight (Wh/kg) than the same. The lithium-ion second battery is now known as one of the lightest power sources and demanded for more improvement.
The lithium-ion secondary battery includes a negative active material of lithium and is thus regarded as a lithium secondary battery. Also, it uses a non-aqueous electrolyte such as organic electrolyte or polymer solid electrolyte and is regarded as a non-aqueous electrolyte secondary battery.
Essentially, the lithium-ion secondary battery comprises a positive electrode made of lithium contained cobalt oxide, LiCoO2, or a double oxide of lithium and cobalt, and a negative electrode made of a carbon material such as graphite or coke. The electrodes are separated by a separator, assembled to form an electrode group, and placed in an organic electrolyte. In use, when the secondary battery is initially charged, lithium ions are desorbed from the positive electrode of LiCoO2 and dissolved into the electrolyte. Simultaneously, lithium ions in the electrolyte are absorbed in the carbon material of the negative electrode to form C6Li. During the initial discharge, lithium ions in the electrolyte are absorbed in the positive electrode and restored to a LiCoO2 form. At the time, lithium ions are desorbed from the negative electrode of C6Li and dissolved into the electrolyte. Because the charge and the discharge reaction on the positive and negative electrodes are reversible, the battery system is called a rocking-chair battery. The rocking-chair battery may have a longer cycling life of over 1000 cycles provided that neither overcharge or over-discharge occurs.
It is, however, said that the reversible reaction on the positive and the negative electrodes in the charge and discharge are not uniform. As described above, the initial charge permits lithium ions to be desorbed from LiCoO2 of the positive electrode, but not the whole amount of lithium ions is absorbed in the positive electrode and restored to LiCoO2 in the initial discharge. In other words, it is common that a smaller amount of lithium ions than the amount desorbed in the initial charge is successfully absorbed in the positive electrode. Also, in the initial charge, an amount of lithium ions equivalent to the charge capacity of the positive electrode is absorbed in the negative electrode made of a carbon material to form to C6Li. The negative electrode however desorbs about 80% or more of the whole amount of absorbed lithium ions in the initial discharge. As the remaining 20% of lithium has been trapped in the negative electrode, it will not participate in the charge and discharge reactions of a succeeding cycle. Such an amount of lithium ions trapped in the negative electrode and isolated from the charge and discharge reactions is regarded as xe2x80x9cdead lithiumxe2x80x9d and should be discriminated from the other active portion. Although the efficiency of reaction during the charge and discharge after the initial discharge is affected by the rates of charge and discharge and the ambient temperature at the site and may not reach 100%, its declination will is not compared to a difference between the initial charge capacity and the initial discharge capacity. It is hence essential to design constitution of the lithium-ion secondary battery to account a ratio of the initial discharge capacity to the initial charge capacity (referred to as an initial charge and discharge efficiency hereinafter) on the positive and negative electrodes in order to determine the theoretical capacity values of the positive and negative electrodes.
The capacity of the secondary battery will be increased when a material having a higher charge-discharge efficiency or, more specifically a higher initial charge-discharge efficiency, is used as the positive and negative electrodes. An example using LiCoO2 as the positive electrode is disclosed in Japanese Patent Laid-open Application No. Sho63-59507. LiCoO2 has a higher initial charge-discharge efficiency and also a higher electrode potential (thus producing a higher voltage output of the battery), hence being suited as a material for the positive electrode. Cobalt is however an expensive material that is produced in only a few regions of the earth (for example, Zambia in Africa). Hence, its supply and price largely depend on the political situation in the regions. It is thus proposed to substitute such a critical material as LiCoO2 with lithium contained in nickel oxide, LiNiO2, which is favorable in both the price and availability, which may provide a higher capacity than that of LiCoO2, as disclosed in Goodenough, U.S. Pat. No. 4,302,518.
The electrode potential of LiNiO2 is about 0.2 volt lower than that of LiCoO2 and may thus promote the desorbing of lithium ions before the non-aqueous electrolyte such as organic electrolyte reaches its decomposition voltage in the charge. This results in increase of the charge capacity and thus improvement of the discharge capacity. However, the initial charge-discharge efficiency of LiNiO2 is not high enough and causes declination of the capacity as the charge and discharge cycle is repeated again and again, whereby its practical use will be difficult.
To eliminate the drawback of LiNiO2, double oxide such as LixNixCo1xe2x88x92xO2 or LixMyNzO2, including lithium and plural metals (where M is at least an element selected from a group of Fe, Co, and Ni, and N is at least an element selected from a group of Ti, V, Cr, and Mn) is provided as disclosed in Japanese Patent Laid-open Application Sho63-299056 or Publication Sho63-267053.
A. Rougiel et al, Solid State Ionics, 90, 83-90 (1996) have reported that LixNixCo1xe2x88x92xO2 phases crystallize in the rhombohedral system with a layered structure. For small amounts of cobalt, i.e., (1xe2x88x92x) is less than 0.2, extra-divalent nickel ions are always present. However, cobalt substitution decreases the non-stoichiometric character of lithium nickelate. For compositions in which (1xe2x88x92x) is greater than 0.3, a pure 2D structure is observed.
To increase the capacity of the lithium-ion secondary battery, it is essential to use proper materials for the positive and negative electrodes to provide a high initial charge and discharge efficiency and as high a reversible capacity in the charge and discharge reactions as possible. It is also desired to minimize the amount of xe2x80x9cdead lithiumxe2x80x9d and the irreversible capacity on the positive and negative electrodes so that the negative electrode is prevented from being overcharged and free from deposition of metallic lithium. If the positive electrode is higher in the initial charge and discharge efficiency than the negative electrode, it will extremely be difficult to design and fabricate an improved secondary battery that satisfies the above requirements.
The addition of metal oxide, such as FeO, FeO2, Fe2O3, SnO, SnO2, MoO2, V2O5, Bi2Sn3O9, WO2, WO3, Nb2O5, or MoO3, to a carbon material for the negative electrode is depicted in Japanese Patent Laid-open Publication Hei7-192727. However, the metal oxide is selected from compounds that can absorb and desorb lithium in reverse relationship during the charge and discharge. Also, the metal oxide is intended for preventing the negative electrode potential to sharply rise in the end of the discharge and dissolve the copper foil as a core material of the electrode, hence decreasing the cycle life. The purpose and the effect of addition of the metal oxide are not equal to those of the invention where metal oxide is carefully selected and used for promoting electrochemical reduction in the initial charge to generate a quantity of metal in irreversible reaction.
The invention is a non-aqueous electrolyte secondary battery. The battery comprises a positive electrode and a negative electrode separated by either a separator impregnated with organic electrolyte solution or by a solid electrolyte layer. The positive electrode comprises mainly lithium containing metal oxide, and the negative electrode comprises mainly a mixture of a carbon material, as the active material, and the metal oxide can electrochemically be reduced to metal by charge. This allows the negative electrode to eliminate xe2x80x9cdead lithium,xe2x80x9d which hardly contributes to the charge and discharge reactions and to be prevented from being overcharged. Accordingly, the non-aqueous electrolyte secondary battery of the invention will increase its cycle life and capacity.
The advantage of the invention is enhanced with the positive electrode made of a lithium containing metal oxide that comprises mainly a lithium containing nickel oxide having 75 to 95% of initial charge and discharge efficiency.